We'll see if it fixes the big issue I'm having - a friend who ditched his iPhone can exchange SMS messages with me and others individually, but he can't participate in group threads anymore. No error, he just never receives the messages.
This is only an issue for me because Apple made the arbitrary decision to make your iPhone functionally useless after a restore unless you insert a SIM. I still use my iPhone for dev. Guess what happened after a restore once I inserted my SIM? My iPhone automatically enrolled my phone number in iMessages. I spent months getting SMS working after switching to Android and I'm back at square one. This is antitrust level BS.
In my experience, it looks like iMessage stores metadata about the chat medium for a given conversation, and that's determined upon creating the conversation.
When sending individual messages, iMessage doesn't seem to attempt to determine whether conversation partner has gone dark from iMessage and fall back to MMS going forward. So everyone else gets the message, but not the person without an iPhone.
The key is that the recipient list is sent with the message, so the other clients know who else is participating in the group, and can also thread and send outgoing messages accordingly.